Lighting apparatuses with light adjusting function have been widely used. For example, a lighting apparatus using an incandescent light bulb is capable of adjusting light by changing the level of current flowing through a filament serving as a light source. In adjusting the light from the incandescent light bulb from a darker state into a brighter state, for example, the emission color of the incandescent light bulb turns from orange into white. This is because the emission color of the incandescent light bulb changes depending on the temperature or the like of the filament, and the color temperature of emission of the incandescent light bulb decreases as the temperature of the filament decreases. The temperature of the filament changes depending on the level of current flowing through the filament.
On the other hand, there has been a recent growing popularity of replacement of the incandescent light bulb with a lighting apparatus using a light-emitting module including semiconductor light-emitting elements such as LEDs. In general, a change in level of current flowing through an LED chip does not change the emission color of the LED chip. This is because the emission color of the LED chip depends on the bandgap of a semiconductor material included in the LED chip, but does not depend on the current level. Hence, replacement of the incandescent light bulb with a lamp using LEDs as a light source (hereinafter, referred to as an LED lamp) in the conventional lighting apparatus having light adjusting function may cause a user to have a feeling of strangeness in regard to the emission color of the LED lamp during light adjustment.
In view of the above, Patent Literature (PTL) 1 discloses an LED module which is capable of changing the emission color in the use of the LEDs.
FIG. 11 is a circuit diagram of a conventional LED module disclosed in PTL 1. As shown in FIG. 11, the LED module 900 includes a red LED array 921 and a white LED array 922 which are connected in parallel. The red LED array 921 includes red LEDs 921a, 921b, 921c, . . . , 921d, 921e, and 921f which are connected in series. The white LED array 922 includes white LEDs 922a, 922b, . . . , 922c, and 922d which are connected in series. The white LED array 922 is connected in series to a bipolar transistor 924 and a resistive element 926. The bipolar transistor 924 has a base terminal connected to a variable voltage source 927 via a resistive element 925. Furthermore, the bipolar transistor 924 has a collector terminal connected to the cathode terminal of the white LED 922d, and an emitter terminal connected to the resistive element 926.
The LED module 900 is connected to a variable current source 933. Alternating-current (AC) power supplied from an AC source 931 undergoes AC to DC conversion performed by an AC/DC converter 932, and the resulting power is supplied to the variable current source 933. Accordingly, current is supplied to the LED module 900 from the variable current source 933.
The LED module 900 is capable of changing base current by changing base-emitter voltage of the bipolar transistor 924. Here, the collector current increases as the base current of the bipolar transistor 924 increases. This leads to an increase in current flowing through the white LED array 922. By increasing the current flowing through the white LED array 922 among the current supplied from the variable current source 933, the current flowing through the red LED array 921 relatively decreases. As a result, the emission color of the LED module 900 approaches white. On the other hand, by reducing the current flowing through the white LED array 922, the current flowing through the red LED array 921 relatively increases. As a result, the emission color of the LED module 900 approaches orange.